Microbes have diverse applications in agriculture to improve yield, nitrogen and phosphorus Absorption, disease and pest resistance, and tolerance to drought conditions.
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The most limiting nutrients in soil for plant growth and development are nitrogen and phosphorus. Therefore, chemical fertilizers are applied to the soil to increase the bioavailability of these nutrients. Due to excessive use of synthetic fertilizers, environmental problems such as the decrease in soil and water quality are caused. Therefore, to increase the bioavailability of N and P, more attention should be paid to microorganisms that can be used in cropping systems.
Phosphorus is the most limiting nutrient of all plant nutrients. The bioavailability of phosphorus in any soil is negligible. Phosphorus solubilizing bacteria and mycorrhizas increase the solubility of phosphorus applied to the soil.
These bacteria and fungi secrete organic acids. Those organic compounds dissolve phosphorus-containing minerals. As a result, phosphorus is released into the soil solution by forming a chelate of phosphate ions. Currently, commercially formulated microbial biofertilizers can be found in the market.
Nitrogen-fixing micro-organisms
Bio nitrogen fixation is the process by which microorganisms convert atmospheric molecular nitrogen into its soluble forms.
Rhizobium forms a close association with certain leguminous plants. Fixed nitrogen is released into the soil after legume plants die and is used by other plants.
Anabaena sp. forms a symbiotic association with the aquatic fern Azolla.
Free-living nitrogen-fixing bacteria such as Azotobacter exist in high concentrations in the rhizosphere.
Many bacteria livings in the rhizosphere of plants produce substances such as auxins, cytokinin, and gibberellins that improve plant growth.
Nature and the role of soil micro-organisms
The majority of soil microorganisms are represented by bacteria, but also fungi, algae, protozoa and actinomycetes. Although the actinomycetes are in the bacterial kingdom, they are usually listed separately because of their importance. These microbes play a major role in the decomposition of complex organic matter and the cycling of elements in biochemical cycles. Microorganisms oxidize and deoxidize elements for their metabolic needs.
The role of microbes in the carbon cycle
All living things contain a large amount of carbon as organic compounds such as cellulose, starch, protein and fat. Photosynthesis is an important key step in the carbon cycle. In it, inorganic carbon dioxide is fixed by Photosynthetic organisms to produce organic compounds. Photoautotrophs such as plants, cyanobacteria, algae, and photosynthetic bacteria fix carbon dioxide by obtaining energy from sunlight.
Chemo heterotrophs such as protozoa depend on organic matter produced by autotrophs for consumption as their organic source. Carbon fixed by autotrophs from carbon dioxide moves from organisms at lower trophic levels to organisms at higher trophic levels along food chains.
Both autotrophs and chemo heterotrophs release some of the fixed carbon into the atmosphere as carbon dioxide through respiration. That process provides carbon dioxide for autotrophs.
The undigested food that is released into the environment as excreta by the chemo heterotrophs is then decomposed by soil microbes. The remaining carbon fixed by organisms remains in their bodies until their death.
After the organisms die, those organic compounds are decomposed, and carbon dioxide is released back into the atmosphere. During the decomposition of organic matter, microbes break down key compounds and release carbon dioxide back into the atmosphere. Microbes, mainly bacteria and fungi, play a major role in the decomposition of organic matter. Microbes play another important role in methane production. Ocean sediments contain large amounts of methane. Microbes known as methanotrophs consume about 80% of the methane produced in the oceans before it is released into the atmosphere.
The role of microbes in the Nitrogen cycle
All living things need nitrogen to make proteins, nucleic acids and other nitrogen-containing compounds. About 80% of molecular nitrogen exists in the atmosphere. They are not bioavailable to living organisms. Therefore, it is necessary to convert the molecular nitrogen into a bioavailable form of nitrogen.
Certain groups of microbes are capable of converting atmospheric molecular nitrogen into usable forms for living organisms as ammonia, nitrate and nitrite. Therefore, the nitrogen contained in the earth's organisms and in the atmosphere must flow in a cyclical manner.
The nitrogen cycle includes four main steps. They are ammonification, nitrification, denitrification, and nitrogen fixation.
Ammonification
More than 90% of the organic nitrogen in the soil is in the form of proteins. The intracellular proteolytic enzymes secreted by microbes break down the proteins of dead plants and animals into amino acids. The resulting amino acids are taken into the microbial cells and undergo ammonification, and the amine group of amino acids is converted into ammonia. In wet soil, ammonia dissolves in water to form ammonium ions. This ammonium is used by plants and soil organisms. Ammonia in dry soil is rapidly released into the atmosphere.
Nitrification
Nitrification is the process by which nitrogen from ammonium ions is oxidized to produce nitrate. This is done in two steps by nitrifying bacteria living in the soil.
In the first step, microbes such as Nitrosomonas oxidize ammonium ions to nitrite. In the second step, microbes like Nitrobacter oxidize nitrite to nitrate.
Plants use this nitrate as their nitrogen source. Therefore, microbes play an essential role in providing bioavailable nitrogen to plants and animals.
Denitrification
Some microbes oxidize nitrate to N2 in atmospheric oxygen-deprived conditions. This process is nitrification. Here nitrogen is released into the atmosphere. Therefore, the amount of nitrogen in the soil decreases. Because oxygen is limited in waterlogged soils, nitrification is constant.
Nitrogen fixation
The process of converting nitrogen gas into ammonia is called nitrogen fixation. Bacteria carrying nitrogenase enzymes carry out nitrogen fixation. These microbial nitrogenase enzymes have mechanisms to prevent exposure to atmospheric oxygen. Nitrogenase is inactivated by oxygen. Free-living and symbiotic are two forms of nitrogen-fixing bacteria. Nitrogen-fixing free-living bacteria are abundant in the rhizosphere. The soil in which plant roots are located is the rhizosphere. Symbiotic nitrogen fixing bacteria are commonly called rhizobia. Legumes are specially adapted to facilitate symbiotic nitrogen fixing.
Soil microbes in the soil interact directly with plants. rhizospheres, mycorrhiza, endophytes are these interactions.
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